1. Field of the Invention
The present invention relates in general to magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS). More particularly, the present invention relates to a universal-tuned RF probe coil used to cover all the applicable nuclei utilized in nuclear magnetic resonance (NMR), such as MRI and MRS.
2. Prior Art & Background Information
A variety of nuclides, such as .sup.1 H, .sup.31 P, .sup.13 C, .sup.19 F, .sup.2 H, .sup.29 Si, .sup.27 Al, and .sup.15 N, may be studied using magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI). Many tools have arisen for studying these chemicals, and applications employing double nuclear resonant circuits (coils), for studying multiple nuclides simultaneously, have been used for the past 30 years. Recently, double resonant circuits have been widely used in MRI and MRS, for clinical and animal experiments.
In U.S. Pat. No. 4,742,304 Schnall disclosed a multi-tuned NMR probe using a trap-matrix approach. In order to achieve multi-tuning of the radio frequency (RF) coil, parallel LC traps were added in series with the tuning capacitor. However, the traps do not contribute to the B1 field at any resonant frequencies. To the contrary, they consume RF energy and add to coil losses, reducing overall coil performance.
A triple resonance or quad resonance NMR probe circuit was described by Doty in U.S. Pat. No. 5,424,645. This probe was designed using two double broadbanded matching networks, and claimed some of the following advantages: that it can actually be built, fit within the space allowed, and handle high power. However, to approach triple or quad RF frequencies, Doty used tank circuits whether in series, parallel or a combination thereof, as an addition to the circuitry. Thus, as in Schnall's design, the RF energy is distributed among different resonant circuits and overall coil efficiency is reduced.
The trap circuit method was also used by Romeo to create double, triple and higher order tuned birdcage volume coil designs. ("Use of Transmission Line Analysis for Mult-Tuning of Birdcage Resonators," Magnetic Resonance Imaging, pp. 705-715, 1993). However, in principle this design is the same as those presented by Schnall and Doty, and has low coil efficiency due to the energy lost in the trap circuits.
Srinivasan disclosed a multi-tuned RF coil in U.S. Pat. No. 5,680,047. In this design, two different frequencies (high or low) are created by using a co-rotating mode and a counter-rotating mode. Although this design has an advantage of not using trap or tank circuits, this design has two limitations: (1) it achieves only two resonant frequencies, and (2) B 1 field patterns are different for the different nuclei, because their current patterns are different (co-rotating mode vs. counter-rotating mode), thus co-registration of spatial data obtained from the two separate frequencies is very difficult.
Another multi-tuned RF coil design has been reported, and is based on a coaxial cavity which is tuned to the desired frequencies by adjusting the impedances of symmetrical groups of transmission line elements (see, "Multiply Tuned, High Frequency Volume Coils for Clinic NMR", Vaughan et al., p. 1119, MRM, 2nd Annual Meeting, 1994). Because all the different modes are dependent each other, the coil tuning and matching procedure would be very difficult in practice. In this design, the RF energy is consumed by the different frequencies which reduces the overall coil performance.
A double-tuned coil using a switching technique is described in "High Efficiency Double-Tuned RF Coil using a Switching Technique", Shen et al., p. 1124, MRM, 2nd Annual Meeting, 1994. This advantage in this design is its very high efficiency for both frequencies, but it is limited by covering a wide range of two frequencies. For example, the resonant frequency, 64 MHz of .sup.1 H is about 10 times of 6.46 MHz of .sup.15 N at 1.5 T. Two capacitors are required, with 100 times difference to cover these two frequencies--which is not practical, because the special non-magnetic capacitors required in MRI and MRS for covering wide ranges are not commercially available.
The switching technique has also been used for automatic tuning as described by Magnuson in U.S. Pat. No. 5,594,338. However, it is limited to a narrow tuning range of "coarse" or "fine" tune for a single-tuned coil case.
Although using a trap or tank circuit method can easily create more frequencies, the coil efficiency decreases with each additional resonant frequency. Other methods, such as designs of current modes, harmonics, or switching may also create more frequencies, but in practice they are very difficult physically implement.
No one has ever reported a radio frequency (RF) coil or probe in MRI and MRS which can cover more than 5 nuclei. Most current multi-tuned coils or probes are less than 4-tuned and have overall low efficiency. It is therefore an objective of the invention to provide a universal-tuned RF probe which can operate independently for each nuclide to optimize coil performance and overcome the previously described problems. It is also an object of this invention to employ a very thin multi-layer coil, to maintain the same B1 field patterns for all nuclei while maintaining the geometry of the coil for purposes of co-registration of data. Thus, the invention not only evaluates multiple nuclei simultaneously through a fast and efficient switching technique, but it allows the sample to be studied in the same field through utilization of the thin multi-layer coil.